This invention relates to feed forward amplifiers for achieving low distortion during signal amplification and in particular to high efficiency feed forward RF amplifiers. It particularly concerns a feed forward amplifier having a closed loop control of the amplifier to counteract time and temperature variations. It specifically involves RF (radio frequency) amplifiers used in wireless applications. Multi-tone DMT (discrete multi-tone) feed forward RF amplifiers are particularly amenable to the feed forward process described.
Feed Forward (FF) amplifiers are special amplifiers that include circuitry for reducing distortion (i.e., in DMT RF amplifiers, intermodulation distortion is caused by the beating of the multi-tone signals together) normally introduced by the amplification process. They (FF amplifiers) are widely used in applications where reduction of signal distortion is critical to achieving performance objectives related to amplified signal clarity. Reduction of distortion is accomplished by use of correction or canceling signals, which are derived from the very signal components causing the distortion and then summed with the amplified signal.
FF RF (i.e., low distortion) amplifiers are very valuable in amplifying multi-tone signals such as are used in high frequency radio signals. Typically, such FF RF amplifiers have two substantially parallel signal processing/amplification paths. A first path (i.e., the amplification path) amplifies the multi-tone signal and in doing so creates inter-modulation distortions (IMD) in the resultant amplified signal. A portion of the multi-tone input signal is separated from the primary amplification path and channeled into a secondary path (i.e., the feed forward path) and combined with the signal as amplified in the first path including the IMD. In the signal combining the IMD signal is amplified to equal the IMD of the amplified signal in the first path. These IMD signals are subtracted from the amplified signal to recover a substantially distortionless amplified multi-tone signal.
While following these generalized principals, practical FF RF amplifiers are considerably more complex in structure and operation. This complexity is needed in part because of the high frequency operation at RF, which accentuates the IMD products. This complexity often takes the form of added control loops or/and added signal processing circuitry. This complexity however often compromises the basic purpose of the FF RF amplifier by introducing inefficiencies, which reduce its overall amplification capability. Hence, it is important to maintain amplification capability as well as reduce IMD.
One example of a widely used FF RF amplifier is described in U.S. Pat. No. 5,917,375 issued to Lisco et al Jun. 29, 1995. This particular FF RF amplifier uses a correction amplifier in a feed forward loop to amplify the signal (i.e., increase output power) as well as providing the IMD cancellation signal to reduce distortion.
Another FF RF amplifier is disclosed in the U.S. Pat. No. 6,111,462, issued to Mucenieks et al on Aug. 29, 2000, uses predistortion in the feed forward path to include the same RF carrier component as that in the amplification path. The amplified signals in both paths are combined (i.e., summed) to produce a high power amplified signal without the IMD components. The circuitry however is complex and power consuming causing inefficiencies. In particular, Quadrature detectors are a required component of the distortion correction circuitry.
High efficiency and high power output are attained in a multi-tone FF RF amplifier by eliminating fundamentals in a first loop and then operating on the IMD products only in a second loop. Since lower power tones are adjusted in the IMD product reduction, less complex, lower power processing circuitry is required and higher efficiency and lower costs are realized.
In an exemplary embodiment a multi-tone high frequency signal is applied to a first main amplification path, which amplifies a multi-tone high frequency input signal. In the process of amplification, by the first main amplification path, IMD is introduced into the amplified signal.
Input to the main amplification path is sensed at the input and channeled into a secondary amplification path (i.e., sometimes known as a feed forward path), which initially delays the multi-tone high frequency signal. This delayed multi-tone signal is combined with a feedback version of the amplified signal of the main amplification path output to add the distortion tones and permit further phase and amplitude adjustment. Phase adjustment is designed to invert the IMD and align the inverted IMD of the first loop path such that the IMD is out-of-phase with the IMD in the secondary path and combines this IMD in the secondary path through a coupler. This secondary amplification path signal combining the IMD and the multi-tone input is amplified so that the inverted distortion terms are equal in absolute value as to the distortion terms of the main amplification path output. With judicious amplitude, and signal delay distortion products (IMD) are canceled at the output and the multi-tone signal amplitude is enhanced by addition of the two amplification path outputs. This results in a clean (i.e., distortion free) multi-tone signal and improved amplifier efficiency due to the additive enhancement of the output multi-tone signal.
It is readily apparent that the forgoing feed forward system cancels IMD as well as increasing multi-tone amplitude by adding signal output at both the primary and secondary amplifiers, providing improved amplification efficiency and improved low distortion.